Posted by **Anonymous** on Wednesday, February 22, 2012 at 4:36pm.

Given that cos2x=7/12 and "270 equal or < 2x equal or < 360", find sinx.

Please help and Thank you

- Trig Help -
**Reiny**, Wednesday, February 22, 2012 at 5:06pm
In these type of quesstions it usually asks for the "exact" value of ....

cos 2x = 7/12

cos 2x = 1 - 2sin^2 x

2 sin^2 x = 1 - cos 2x = 1 - 7/12 = 5/12

sin^2 x = 5/24

sin x = √(5/24)

but 270 < 2x ≤ 360

135 ≤ x ≤ 180 ---> x in in I or II, so

sinx = +√5/√24 = √30/12

- Trig Help -
**Anonymous**, Wednesday, February 22, 2012 at 5:49pm
how did you get the 30/12?

- Trig Help -
**Anonymous**, Wednesday, February 22, 2012 at 6:15pm
sqrt(5)/2sqrt(6)

when you rationalize the denominator you times both top and bottom by sqrt(6) only not the 2 right?

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